The purpose of this research is to examine the ultrastructure of cellular differentiation in the human fetal ovary and testis. We will determine the specific changes occuring in germ cells, "supporting" (Sertoli, granulosa) cells, steroid-secreting (Leydig, interstitial) cells, and surface epithelial cells at different stages of development. Beginning with gonadal differentiation at 6-7 weeks, this investigation will extend as far as the 20th week gestation period. Special techniques to be employed include lanthanum tracer studies to indicate the development of cellular junctions in the sex cords and interstitial tissue, and scanning electron microscopy to define the topographical relationships within the surface epithelium and developing sex cords. Our morphological findings will then be compared with corresponding age-related data on steroid and gonadotrophic hormone levels in the human fetus to identify structure-function correlations. The preparation of an atlas on the ultrastructure of human gonadal development is a major goal of these studies.